The vitamin D metabolic pathway is part of a vital endocrine system that is highly regulated at certain stages and produces metabolites that control the secretion of the parathyroid gland hormones (Beckman, M., and DeLuca, H. (1997) Methods in Enzymol. 282, 200-223; Jones, G., Strugnell, S., and DeLuca, H. (1998) Physiol. Rev. 78, 1193-1231). 1α,25-Dihydroxyvitamin D3, also known as calcitriol (see below), a hormone produced in the vitamin D pathway, regulates phosphate and calcium levels in the blood which in turn control bone mass, the state of bones, and affects cellular differentiation in the skin and the immune system (Armbrecht, H. J., Okuda, K., Wongsurawat, N., Nemani, R., Chen, M., and Boltz, M. (1992) J. Steroid Biochem. Molec. Biol. 43, 1073-1081). In the vitamin D pathway, cytochrome P450s are enzymes that introduce functional groups by hydroxylation, usually at positions 1, 25, and 24, of vitamin D3 (Beckman, M., and DeLuca, H. (1997) Methods in Enzymol. 282, 200-223).

1α,25-Dihydroxyvitamin D3 is converted to 1α,24,25-trihydroxy-D3 by a mitochondrial P450 known as CYP24 (Bell, N. H., (1998) J. Bone Miner. Res. 13, 350-352). CYP24 is induced by 1α,25-dihydroxy-D3 and is found in the kidney as well as other vitamin D target tissues such as the parathyroid cells, keratinocytes, osteoblasts, and enteroctyes (Jones, G., Strugnell, S., and DeLuca, H. (1998) Physiol. Rev. 78, 1193-1231).
The biological effects of 1α,25-dihydroxyvitamin D3 (calcitriol) and its synthetic analogs are mediated by the nuclear vitamin D receptor (VDR). Calcitriol has an important role in the antiproliferative and growth regulatory effects on normal and neoplastic cells (for e.g. prostate cancer cells). VDR ligands have potential widespread clinical application, however in many cases, hypercalcemia develops as a side effect which prevents sustained systemic administration. Inhibiting the catabolism of calcitriol and its analogs is expected to lengthen the biological lifetime of these compounds and thus to allow smaller amounts of them to be used for effective human chemotherapy. Such smaller dosing will avoid, or at least minimize, the hypercalcemic toxicity associated with medicinal use of these compounds. Further inhibition of the catabolism of 1α,25-dihydroxyvitamin D3 increases the endogenous levels of this hormone, which will also have beneficial therapeutic effects.
There is a need for compounds that mimic and modulate the activity of calcitriol, and analogs thereof, yet do not possess undesirable side effects, for example, hypercalcemia.